1. Field of the Invention
The present invention relates to a voltage controlled oscillator capable of switching an oscillation frequency, and further to an effective technique for improvement in characteristic of a voltage controlled oscillator integrated into a chip. The present invention is directed to expanding a frequency variable range and providing a voltage controlled oscillator that has a stable and sufficiently small phase noise characteristic regardless of an oscillation frequency.
2. Description of the Related Art
With the recent increase in integration of a semiconductor integrated circuit for communication, a phase noise of a voltage controlled oscillator (VCO), having hitherto been constituted of an outboard module, is attempted to be lowered by improvement in a Q value of a spiral inductor produced on an on-chip, and by provision of a capacity band switching circuit as shown in a VCO of FIG. 9 to lower a change in frequency (hereinafter referred to as VCO sensitivity) with respect to a control voltage VT. Thereby, the integration of the voltage controlled oscillator into a chip has been realized (eg. Cf. Japanese Patent Laid-Open No. 2001-339301)
The VCO of FIG. 9 comprises: a resonance tank circuit A, comprising reactors L1 and L2, fixed capacities C1 and C2, a pair of variable capacity diodes VC1 and VC2 anodes of which are grounded, and negative resistors R1 and R2 connected respectively to the respective cathodes of the variable capacity diodes VC1 and VC2; an amplifier B comprising N-MOSFETs N1 and N2 for positive feedback (oscillation) amplification with an oscillation frequency of the resonance tank circuit A; and a current mirror circuit C comprising transistors Q1 and Q2 for feeding a constant current IVCO to N-MOSFETs N1 and N2.
By a change in control voltage VT to be applied to the respective cathodes of the variable capacity diodes VC1 and VC2 via the negative resistors R1 and R2, capacity values of the variable capacity diodes VC1 and VC2 change, and thereby the oscillation frequency changes in the resonance tank circuit A.
Further, the VCO of FIG. 9 comprises a capacity band switching circuit D. After capacity band switching capacities C3, C4, C5, C6, C7 and C8 are selectively added by the on/off of respective switches SW, an oscillation frequency can be changed in each of different frequency bands by control of the control voltage VT. The oscillation frequency fVCO of the VCO of FIG. 9 is given by equation 1. It is to be noted that the constitution of this capacity band switching circuit D is specifically described later.fVCO=½π√{square root over ( )}L·Ct  (1)where Ct is a total sum of capacity components on the actuating piece side of the voltage control oscillator, and when all the switches are on,
Ct=parasitic capacity+C3+C5+C7+C1·VC1/(C1+VC1)
Here, in the case where the frequency characteristics of the oscillation transistors N1 and N2 are sufficiently high, the lower the oscillation frequency fVCO becomes, the larger the capacity value needs to be made, as seen from Expression 1. This therefore means that the voltage controlled oscillator is in operation in the state of having a large parasitic capacity. When a voltage controlled oscillator, designed with a certain frequency as an operational frequency, is used with a frequency lower than the oscillation frequency used in the designing, an output amplitude decreases to cause a phase noise to increase. Further, as opposed to this, when a voltage controlled oscillator, designed with a certain frequency as an operational frequency, is used with a frequency higher than the oscillation frequency used in the designing, a phase noise increases under the influence of an excess current noise and because the output amplitude increases to exceed the optimum operational range of the transistors N1 and N2.
On the other hand, in the case where the frequency characteristics of the oscillation transistors N1 and N2 are low, when a voltage controlled oscillator designed with a certain frequency as an operational frequency is used with a frequency lower than the oscillation frequency used in the designing, driving capacities of the oscillation transistors N1 and N2 become more dominant than in the state where the parasitic capacity is large, to cause an output amplitude to increase, leading to reduction in phase noise. As opposed to this, when a voltage controlled oscillator, designed with a certain frequency as an operational frequency, is used with a frequency higher than the oscillation frequency used in the designing, the frequency characteristics of the oscillation transistors N1 and N2 decrease, and due to insufficient driving capacities, an output amplitude is reduced, leading to an increase in phase noise.
FIG. 10 shows actual measurement values of the phase noise characteristics in the respective cases where the frequency characteristic of the oscillation transistor is sufficiently high and low in the conventional circuit of FIG. 9.
As described above and seen from FIG. 10, in both the cases where the frequency characteristics of the oscillation transistors N1 and N2 are sufficiently high (shown by a solid line) and low (shown by a broken line), in a voltage controlled oscillator designed with a certain frequency as an operational frequency, an increase in phase noise or stopping of oscillation in a narrow band may be incurred if a frequency other than the frequency used in the designing is oscillated. It has therefore been difficult to integrate a voltage controlled oscillator into a chip in a communication system requiring a strict phase noise characteristic, such as GSM (Global System for Mobile Communication), and to realize one on-chip voltage controlled oscillator in a frequency band using two systems such as GSM and DCS (Digital Cellular System).
For solving this problem, there is a method of making an inductor externally placed to increase a Q value, as well as a method of using a mask option to increase a Q value of a spiral inductor or a Q value of a capacity, so as to reduce a phase noise. However, this method has a problem of causing an increase in production cost.
The present invention is solving the above-mentioned conventional problems, to expand a used frequency band and provide an on-chip voltage controlled oscillator into which a favorable phase noise characteristic in a broad band is integrated at low cost.